The present invention relates to a system ensuring automatic positioning of a bit-carrying carriage at the reference position of a machine using a numerical control system.
When an automatic tool changer or automatic pallet changer is mounted on a numerically controlled machine tool, or when the zero point of program is to be provided, in order that its function be performed, it is necessary for the carriage of the machine be positioned at the reference position of the machine. Automatic positioning is also necessary when the power switch of the numerically controlled machine is initially turned on.
For example, in the case of a machining center with three spindle controls, X, Y, and Z, and also in the situation of positioning the automatic tool changer at the zero point, the spindles Y and Z are to be positioned at their respective zero points or reference positions. Also, when an automatic pallet changer is to be positioned at the zero point, the spindle X must also be positioned at its zero point. Thus, in a prior art, each spindle usually had to be positioned at its respective zero point or reference position. The following is a description of a system of positioning at the zero point of the machine tool, using a conventional numerical control system with reference to FIGS. 1, 2, and 3. For the sake of simplicity, a description is provided for one spindle, but it will be understood that the same principle applies to all other spindles.
FIG. 1 illustrates a simplified block diagram of a numerical control device with one spindle. In the figure, the reference numeral 11 is the NC data input unit that transmits NC data via paper tape or push-buttons for positioning at the zero point of the machine. Numeral 12 is the control device into which the NC data and the signals from the location sensor 17 and from the location detector 18 are input. The sensor 17 is called the first sensor and operates as a coarse sensor; the detector 18 is called the second sensor and operates as a fine sensor. The control unit is connected to the motor drive unit 13. Numeral 14 is the motor that drives the carriage 16. The sensor 17 transmits a binary code to the control unit 12 upon detecting the traveling condition of the carriage 16 in relation to the fixed machine bed 15. If a micro switch is provided on the sensor 17, the output signal of the sensor 17 will be either an ON or OFF signal.
The location detector 18 (second sensor) detects the instantaneous location of the carriage 16 and transmits a signal to the control unit 12. The location detector 18 can be, for instance, implemented by an inductosyn, manufactured by Inductosyn Corporation of U.S.A. The inductosyn has a fixed printed pattern having 2 mm pitch and a slidable portion having 3 kHz triangular wave generator. The inductosyn compares to the phase on the fixed portion with that of the slidable portion and permits fine detection of the instantaneous location. It should be noted that the inductosyn cannot provide the absolute value of the location, but can only detect the relative location within a given scale.
FIGS. 2 and 3 illustrate the conventional positioning system. In those drawings, 19a and 19b are platforms provided on the machine beds 15 close to the zero point or the reference position of the machine. The platform, which is sometimes called a "dog", projects from the fixed bed of the machine. It functions to change the output of the sensors, 17, 17a and 17b located underneath the carriage 16. At the end of the machine bed 15, a wall 20 is provided to operate as a stroke-end for the carriage 16. The device in FIG. 3 has two sensors 17a and 17b instead of a single sensor 17 as in FIG. 2; these two sensors 17a and 17b operate in sequence as the carriage 16 travels.
Because the carriage 16 is far from the zero point of the machine, as shown in FIG. 2, there is no signal from the sensor 17. When the positioning NC data is applied from the input unit 11, the control unit 12 commands the drive unit 13 to drive the carriage 16 at rapid speed in the direction of the arrow as programmed or as instructed by the positioning NC data. When carriage 16 travels, the platform 19a is effective to cause the sensor 17 to output the signal "ON", indicating that the carriage 16 is close to the previously mentioned zero point. The control unit 12 then commands the carriage 16 to travel in the same direction at slow speed. The carriage 16, which has been traveling at rapid speed, overruns by inertia, even after it has been switched to slow speed. However, this force is absorbed during the course of slow-speed operation. As the carriage 16 travels past the platform 19a, in the same direction at slow speed, the output signal of the sensor 17 changes to "OFF", and the control unit 12 then recognizes that the carriage 16 is positioned at the reference pitch point. The above positioning procedure using a platform or a dog is the so-called coarse positioning. After a coarse positioning is finished, fine positioning is performed using the inductosyn. In FIG. 2, the reference pitch point is established at a point just past said platform 19a and is taken as the zero point or the reference position of the machine.
In the rapid traveling phase, the carriage moves, for instance, at a speed of 10 meters per minute, and in the slow traveling phase the carriage moves, for instance, at a speed of 0.18 meters per minute. Coarse positioning establishes the position to within 0.2-0.3 mm. This error is inevitable because of the use of a rough micro switch as a sensor. Fine positioning, employing the inductosyn, is performed after coarse positioning, the error of which is, as already noted, within a pitch length of the location detector (inductosyn) (for instance 2 mm). The fine-positioning error is 0.01 mm or less.
In FIG. 3, because the carriage 16 is far from the zero point of the machine, there is no signal from either of the sensors 17a or 17b. When said positioning NC data is transmitted from the input unit 11, the control unit 12 issues a command as in the case of FIG. 2, and the carriage 16 travels in the direction of the arrow at rapid speed. The signal from the sensor 17a then changes because of the platform 19b. As in the case of FIG. 2, the carriage 16 now travels in the same direction, but at slow speed. As the carriage 16 continues to travel at slow speed, the signal of the sensor 17b eventually changes to "ON". By this signal, the control unit 12 recognizes that the carriage 16 is positioned at the reference pitch point of the location detector 18. After that, of course, fine positioning is accomplished using the location detector (inductosyn).
In FIGS. 2 and 3, when the initial status of the carriage 16 provides an "ON" signal via the sensors 17 and 17a respectively, and when in both cases the positioning command has been transmitted from the control unit 12, the carriage begins to travel at the aforementioned slow speed.
It should be appreciated that, when the machine is initially connected to a numerical control system, an arrangement is made so that the zero point of the machine and the aforementioned reference pitch point (which is coincidentally determined as one and the same point) are made to coincide with each other. Thus, positioning at the zero point of the machine is identical to positioning at the reference pitch point of the location detector.
FIG. 4 shows the block diagram of the control unit 12, which operates in accordance with FIG. 2. In FIG. 4, the terminal T.sub.1 receives the command signal from the input 11, the terminal T.sub.2 receives the output signal from the sensor 17, and the terminal T.sub.3 receives the reference pitch signal from the inductosyn (not shown). The reference numeral 111 designates a decode circuit, 112 and 127 are flip-flops, 114, 115, 116, 117, and 125 are AND circuits, 113 and 126 are NAND circuits, 188 is a rapid-traveling circuit, 119 is a slow-feed circuit, 120 is a fine-positioning circuit, 121 is a direction-decision circuit, 122 is a fixed-direction circuit, and finally, 124 is an OR circuit. When the carriage 16 is at the position shown in FIG. 2, the signal from the sensor 17 is OFF. In this status, when the positioning command is applied to the terminal T.sub.1, the decode circuit 111 decodes the command and sets the flip-flop 112, and then the NAND circuit 113 and AND circuit 114 provides the output signal. Since the flip-flop 127 is in the negative condition at this stage, the AND circuit 117 provides output and causes the rapid-traveling circuit 118 to provide an output signal, which is applied to the drive circuit 13 through the OR circuit 124. In this situation the AND circuit 116 does not provide output; the NAND circuit 123 does provide output, and thus, the high-speed moving direction is decided by the output of the fixed-direction circuit 122. Accordingly, the carriage 16 always moves in a fixed direction (in the left-hand direction in FIG. 2) at rapid speed.
When the sensor 17 is engaged with the platform (or dog) 19a the output of the sensor 17 becomes "ON", the output of the AND circuit 114 disappears, and instead, the output of the AND circuit 115 is provided; the slow-feed circuit 119 then moves the carriage 16 at slow speed. The moving direction in the slow-feed phase is also determined by the fixed-direction circuit 122, thus the carriage moves in the left-hand direction in FIG. 2. Furthermore, the output of the AND circuit 115 sets the flip-flop 127. Next, when the output of the sensor 17 becomes "OFF" again, the NAND circuit 126, the AND circuit 125, and the AND circuit 116 provide output in sequence. The fine-positioning circuit 120 and the direction-decision circuit 121 are thus triggered to operate the location detector (inductosyn) for fine positioning. The flip-flop 127 is reset by the reference pitch signal from the inductosyn through the terminal T.sub.3.
The prior art in FIGS. 2 through 4 has, however, many disadvantages. In the art illustrated in FIG. 2, when said positioning command has been transmitted in a status where the carriage 16 is near the stroke-end 20, (beyond the platform 19a) the carriage 16 collides with the sroke-end 20 at a rapid traveling speed, resulting in breakage of the machine, since the signal from the sensor 17 is "OFF" and the moving direction of the carriage is predetermined by the fixed-direction circuit 122 (FIG. 4). If the platform 19a for the reference position is established near the middle of the entire stroke of the carriage 16, the above accident will occur frequently, making the machine increasingly susceptible to failure. With reference to the art illustrated in FIG. 3, when the carriage 16 has traveled up to the status where the sensor 17b transmits the signal "ON", transmission of the said positioning command causes the carriage, traveling toward the stroke-end 20, to be positioned at a point several pitches ahead of said zero point in terms of the reference pitch of the detector 18. Furthermore, if the slipped-off position is even beyond the stroke-end 20, the carriage will collide with the stroke-end 20. If the zero point is established near the middle of the entire stroke of the carriage 16, the machine becomes more susceptible to this accident. As a result, positioning is set at a slipped-off point a few pitches (in terms of the reference pitch of the location detector) ahead of the real zero point. In addition, two sensors 17a and 17b are required for each spindle, resulting in a total of 6 for control of 3 spindles.
It should also be appreciated that the latest machines have the reference position of the carriage near the center of the entire stroke of the carriage, in order to reduce cutting error. In a prior machine the reference position is located near the end of the stroke of the carriage; as a result the weight of the machine and/or the carriage is not balanced at the reference position, and the change due to the unbalanced weight causes considerable cutting error.